The invention relates to tamper-resistant fasteners and in particular to a tamper-resistant fastener of the type described and claimed in the U.S. Pat. No. 4,258,596.
The fastener described and claimed in the aforesaid patent has a shank portion and a head portion having three sockets in vertical arrangement, one below the other. Each socket is of circular shape. The upper and lower sockets are concentric with the shank. The middle socket is eccentric. The sockets have progressively smaller diameters, the smallest being the lowermost.
The outer sidewall of the fastener head is preferably sloped to make it more difficult for an unauthorized person to apply torque to the fastener to remove it. The head of the fastener is attached to a conventional shank which may be threaded and used in a normal manner.
In U.S. Pat. No. 4,258,596, a special cylindrical drive tool is described and claimed for applying torque to the tamper-resistant fastener. The tool shown in FIG. 3 of the said patent resembles a screw driver in general appearance but, unlike a screw driver, the shank of the tool has a cross-section which is made to correspond to that of the upper socket of the fastener, and the end of the shank has two projections in series. The first projection corresponds in cross-section and position to the middle socket in the fastener. Thus, this projection is eccentric. The second projection corresponds in cross-section and position to the lower socket. Thus, this projection is concentric.
The cylindrical tool mates firmly with the fastener head and, when torque is applied to the handle of the tool, the eccentricity of the first projection causes the projection to engage the wall of the middle socket and to apply torque thereto to turn the fastener. The upper and lower concentric sockets substantially eliminates the tendency of the driving tool to tilt when torque is applied.
While the foregoing is a brief description and summarizes that which is contained U.S. Pat. No. 4,258,596, the entire specification of U.S. Pat. No. 4,258,596 is incorporated into the present application by reference.